1. Field of the Invention
This invention generally relates to the field of touch panel, and more particularly, to a device and method for preventing the influence of conducting material from point detection of projected capacitive touch panel.
2. Description of the Prior Art
Nowadays, the common touch technologies used by electronic devices include resistive, surface capacitive, projected capacitive, surface acoustic wave, optics imaging, infrared, bending wave, active digitizer, and so forth. Since the packaging volumes of the first three technologies are smaller, their precision can be done relatively high. And they are suitable to those smaller mobile device or portable consumer electronic products.
In terms of resistive touch technology, the techniques from pressing a touch screen to the contact detection, data operations, and the contact position confirmed have the technical limitations from the physical conditions. That is, in order to increase the detection area or resolution, it is necessary to increase the number of lines. However, the increase in the number of lines means that the data is also related increase in processing and computing. This causes a heavy load to the processor. In addition, touch-pressure mechanism is confirmed by the mechanical action completely, a PET film, no matter how to improve its pressure-resistance, wear-resistance, anti-deformation and so on, after all, the PET film has its limits. So the performance of the transparency is getting worse with the use of time and frequency. As for contact detection, some specific areas will be worn by excessively use, and thus, the conduction efficiency of an ITO conductive film is reduced. Besides, the ITO conductive film must reserve borders, and thus the optional of the industrial design is restricted. Still, the resistive touch technology is unable to achieve approach sense (fingers approach but not touch), as well as more difficult to deal with multi-touch.
Surface capacitive touch technology does not have to use the ITO conductive film with high-precision, so the touch side has no the similar mechanical structure like the resistive touch technology has. Thus, a surface capacitive touch screen will not be worn nor has a similar touch-mechanical fatigue which results in the sensitivity drop, and can also perform approach sense. However, the surface capacitive touch technology has the problem of hand-shadow effect. That is, when a surface capacitive touch screen is active, if user's wrist and fingers approach the screen surface together, it will make the surface of the ITO conductive film and the inside of the screen generate excessive charges. These excessive charges lead to produce coupling capacitance and make the surface capacitive touch screen sense error. Also, because the surface capacitive technology senses the contact by the change of the electric field, the accuracy of contact detection will be affected when the use of environment has the problem of electromagnetic interference. Still, after a prolonged use, the contact detection also easily offset, so regular or frequent calibration is required.
Referring to FIG. 1A, a three-dimensional decomposition diagram of a well-known projected capacitive touch panel 100 is depicted. The projected capacitive touch panel 100 at least includes a substrate 110, a first sensing layer 120, a dielectric layer 130, a second sensing layer 140, a bonding layer (not shown), and a protecting layer (not shown) from bottom-up to stack up with the same shape. Herein, these elements mentioned above are transparent. The first sensing layer 120 has a plurality of first patterned electrodes 122 serially connected by a plurality of first axial conductive lines 124 correspondingly, and then electrically connected to a plurality of first outside-connection conducting wires 126 correspondingly. The second sensing layer 140 has a plurality of second patterned electrodes 142 serially connected by a plurality of second axial conductive lines 144 correspondingly, and then electrically connected to a plurality of second outside-connection conducting wires 146 correspondingly. In the present diagram, the axial direction of the first axial conductive lines 124 is Y-axial and the axial direction of the second axial conductive lines 144 is X-axial, but not limited to, the first axial direction could also be X-axial and the second axial direction could be Y-axial as well.
Referring to FIG. 1B, the active circuit 150 of the projected capacitive touch panel 100 shown in FIG. 1A is depicted. A plurality of first and second outside-connection conducting wires 126, 146 electrically connect to a sensing unit 160. The relations among the first and the second patterned electrodes 122, 142, the first and the second axial conductive lines 124, 144, and the first and the second outside-connection conducting wires 126, 146 are described in FIG. 1A, and shall not be repeated here. When the circuit is active, the sensing unit 160 sequentially provides a sensing signal to every first axial conductive line 124 by each corresponding first outside-connection conducting wire 126, and then sequentially provides the sensing signal to every second axial conductive line 144 by each corresponding second outside-connection conducting wire 146. In the meanwhile, the first and the second axial conductive lines 122, 144 which do not receive the sensing signal are electrically connected to ground or a fixed voltage level. Since the stray capacitance exists between the first and the second axial conductive lines 124, 144, when a user uses his/her finger or conducting material to approach or touch a touch point TP on the projected capacitive touch panel 100, the finger or the conducting material on the touch point TP forms an extra capacitance with the first and the second axial conductive lines 124, 144. This causes the value of the equivalent capacitance to be changed. The sensing unit 160 senses the relatively bigger change of corresponding current or charges to decide the position of the touch point, such as (X3, Y5). In short, the measuring control circuit sequentially drives a sensing signal to each first and second axial conductive line, and senses the relatively bigger change of corresponding current or charges generated by driving the sensing signal to decide the position of the point. Wherein, when any axial conductive line is driven by the sensing signal and is sensed to get its current change or charge change, other axial conductive lines are electrically connected to ground or a fixed voltage level to make the effect of stray capacitance consistent.
However, when the projected capacitive touch panel 100 has a conducting material area OZ on, such as water or other conducting material, the equivalent circuit and the equivalent stray capacitance between the axial conductive lines on the conducting material area OZ will be changed. This change makes the measuring control circuit, such as sensing unit 160, sense the current change or charge change on the axial conductive lines, and then results in misjudgment and mal-operation. Or, when the axial conductive line related to the touch point TP is provided the sensing signal and is sensed change in current or charges, the current change or the charge change are affected by the conducting material area OZ. That is, those relatively bigger changes of the current or charges are bypassed to the adjacent axial conductive line to ground through the conducting material area OZ. Therefore, the position of the touch point TP cannot be correctly sensed.
In view of the drawbacks mentioned with the prior art of touch point detection, there is a continuous need to develop a new and improved device and method for touch point detection that overcomes the shortages associated with the prior art. The advantages of the present invention are that it solves the problems mentioned above.